The invention relates to an efficient and economical process for treating recovery boiler emissions comprising particulates such as sulfur oxides, hydrogen sulfides and organic sulfur compounds from an industrial gas stream. More particularly, the invention is an improvement of the process disclosed in my U.S. Pat. No. 4,049,399, issued Sept. 20, 1977, entitled "Treatment of Flue Gases."
Mixed emissions of this type are typically found in Kraft and sulfite recovery processes in the pulp and paper industries. For example, emissions from Kraft recovery boilers generally consist of hydrogen sulfide and organic sulfur compounds (designated TRS for total reduced sulfur), SO.sub.2 and particulates. The organic sulfur compounds normally consist of mercaptans, such as methyl mercaptan; mercapto ethers, such as dimethylsulfide; and disulfides, such as dimethyl disulfide. Some references also indicate the presence of carbonyl sulfide (not considered part of TRS).
The quantity and composition of the emissions are a function of boiler feed and loading, boiler operation and process sulfidity. Typical compositions of such boiler emissions are set forth in my referenced patent.
Briefly, in my prior invention, the emissions were treated in a two-stage process to remove the particulates, etc., and the removed materials consisting of salts of the acid gases and captured particulates were returned to the pulping operation at a concentration of about 20% dissolved solids. Before this stream could be reused in the recovery boiler, it had to be concentrated in pre-evaporators to approximately 60% solids.
In the initial operation of my patented process, the effluent stream to be processed passed through an electrostatic precipitator, into a low energy venturi section and was subsequently scrubbed in a packed section. A liquid slipstream containing in the order of 500-3000 lbs/hr of dissolved solids at a concentration of .perspectiveto.20% was realized.
In the practice of the inventive process, it was found that the system efficiency for particulate collection improved with increased solids inlet loading to maintain a constant particulate outlet level. Thus, as a practical matter, the use of the electrostatic precipitator was eliminated. However, with no electrostatic precipitator, this resulted in a system discharge to the pre-evaporators of a 20% solution containing in the order of 10,000 lbs/hr of dissolved solids as compared to 500-3000 lbs/hr when the electrostatic precipitator was used. To concentrate this stream from 20% to 60% created a stream equivalent demand approximately seven times greater than when the electrostatic precipitator was used. Further, the high concentration of sodium sulfate in the slipstream resulted in a deposition of sodium sulfate in the evaporators thus increasing maintenance costs. Because of the high efficiency of the process per se, which functioned more effectively at a high loading inlet level of particulate material, the economic and operational advantage of by-passing the electrostatic precipitator resulted in a deterrant in the economic and operational performance of the pre-evaporators associated with the recovery boiler.